In machining operations, a cutting tool is subjected to stress and elevated temperatures resulting from friction and metal shear. Under such conditions, the cutting edge undergoes continuous change due to wear. Several different concurrent wear processes and their severity determine the tool life. The dominant wear mode of the tool varies depending on the environment which in a broader sense includes all conditions of use (workpiece material, stress, atmosphere, etc.).
In practice, based on extensive testing of Al.sub.2 O.sub.3, Al.sub.2 O.sub.3 --TiC, Si.sub.3 N.sub.4 and Si.sub.3 N.sub.4 --TiC cutting tools, it has been found that ceramic tools during gray cast iron machining wear primarily by abrasion.
Material removal by fracture that occurs in abrasion can be assumed to take place when lateral cracks of adjacent indentations caused by penetration of sharp surface protrusions (or abrasive particles as in pin and disc wear tests) of the opposing surface intersect. The removed volume (V) is then: EQU V.sub.i =r.sub.i h.sub.i l.sub.i
where r.sub.i is critical indentation separation, h.sub.i is the depth of the indentation, and l.sub.i is the sliding distance.
Considering the dependence of the size of the indent and the length of cracks emanating from such angular indentations on the hardness (H) and fracture toughness (K.sub.IC), respectively, the following expression for maximum volume removed by the system of indentors in a grinding operation was derived by Evans and Wilshaw; Acta. Met., 24, 939-956 (1976) ##EQU1## where N is the number of abrasive particles and P is a vertical force on the particle. From experimental wear data obtained using a pin-on-disc method under constant load, the abrasion resistance for a series of Al.sub.2 O.sub.3 and Si.sub.3 N.sub.4 -based ceramic cutting tool materials was found to be directly proportional to K.sub.IC.sup.3/4 H.sup.1/2. This abrasive wear resistance parameter, expressed as the inverse of volume removed per unit length of travel, provides a relative ranking of materials.
A silicon nitride (Si.sub.3 N.sub.4) based composite material containing thirty volume percent titanium carbide (TiC) is presently being marketed by GTE as a metal cutting tool under the name Quantum 5000. The TiC is added to increase the hardness of the composite compared to Si.sub.3 N.sub.4 and thus increase its abrasive wear resistance. See also, U.S. Pat. No. 4,333,979 to Sarin et al.
In many metal cutting operations, the high rate shear strain which separates the chip from the workpiece is converted into thermal energy and high cutting temperatures (in the range of about 800.degree.-1200.degree. C.) are generated. Therefore, the mechanical properties at high temperature are important in determining cutting tool performance and abrasive wear resistance.
Due to the temperatures generated at the cutting edge of a metal cutting tool, the high temperature hardness of the tool material is a critical factor in the tool performance. Improvement in the high temperature hardness of a Si.sub.3 N.sub.4 --TiC material by the addition of a carbide phase with improved high temperature hardness can result in an improved cutting performance when used as a cutting tool material.